Fully expand templates before scanning banner for fields

The behavior when rendering banners for delivery is to look up each
field according to the current banner name.  Therefore, the same
behavior should be used in the admin UI to display all translatable
messages from transcluded templates.

This patch should make it easier to code banners using templates.

GOTCHA: new messages in transcluded templates will not be displayed
by the banner translation UI until the next time banner text is saved.

Change-Id: I55ec8df38081ffbf4248d952b68bd8d9e6efd172